One aspect of the present invention relates to baculoviruses which are known to have insecticidal activity. Their commercial utility has been limited though because, without special formulation, they are readily deactivated by the UV radiation in sunlight. The incorporation of UV protectants in a variety of ways is known and some can enhance the stability of these viruses somewhat, however such compositions can be complicated, expensive to prepare and further improvements in stability and compatibility with the virus are needed.
One method of stabilizing the virus is to mix a water soluble UV absorbing substance with the virus in a spray tank. One such material is a sulfonated copolymer of catechin and leucocyanidin (U.S. Pat. No. 4,094,969). Although some degree of stabilization is achieved, there is no bond between the virus and the protecting substance hence rainfall or dew may readily separate them. Most simple tank mixes of ingredients suffer from this general problem and do not offer sufficient crop protection due to the lack of UV stability of these viruses.
Microencapsulation using a variety of techniques is another method used. However these systems are limited due to the virus's sensitivity to many encapsulating precursors and the overall expense of the preparations. Many, owing to their transparency in the UV-A and -B region of sunshine, require an additional light screening agent. Typically the screening agent is incorporated into the capsule wall such as in U.S. Pat. Nos. 4,844,896 and 4,948,586; EP 0 653 158 A1; and WO 96/03041. A major problem with these preparations is the lack of a suitable triggered release matrix. In other words, the encapsulated virus is constrained in a matrix which requires some process to occur for it to be released at the site of action. These release processes typically include hydrolysis, desorption or dispersion and by their nature take time to occur. Due to the limited residence time of the virus in the insect's midgut, the site of infection, fast and complete release is necessary to reach the full infective potential of the virus
The use of mineral UV screens has also been described Most do not absorb UV light but rather function by reflecting and refracting UV light. Major exceptions to this generalization are zinc oxide and titanium dioxide which have strong UV absorption bands. Although titanium dioxide (TiO.sub.2) is claimed in Japan [WPI Acc. #90-228659/30, J Econ. Entomol. 69, 731(1976), and UK Patent Appl 2,043,4481] to enhance virus stability toward UV radiation, it is also known that when exposed to ultraviolet light from sunlight it becomes reactive. For example, a sunlight irradiated aqueous suspension TiO.sub.2 produced hydroxyl radicals which inactivated viruses in waste water treatment [Water Research 29 (1) 95, 1995].
It has been found that a modified form of TiO.sub.2, referred to as durable TiO.sub.2 (as illustrated in U.S. Pat. No. 4,125,412, EP 0 654 509 Al), provides a better and easier means of stabilization of baculoviruses than known stabilizers. In addition, other advantages of the use of durable TiO.sub.2 will become apparent hereinafter.